The present invention relates to mounts for precision optical elements, and in particular, to a spherical retro-reflector three-point conical nest sphere mount, having a negative acceptance angle.
Modern manufacturing technology, particularly that associated with the construction of large composite material structures, requires the contours of components and tooling to be accurately measured and inspected for compliance with design models and specifications. One manner in which these structures are measured is with the aid of portable coordinate measuring machines such as laser trackers.
Laser trackers measure and inspect large components by illuminating one or more retro-reflecting targets mounted to the components in predetermined locations with a laser. The distance between the laser tracker and the retro-reflector target is measured using the laser, and compared with design models or specifications, such as those stored on a computer.
To facilitate the use of laser trackers, retro-reflectors are centrally mounted in hollow steel balls, commonly referred to as Spherically Mounted Retro-reflectors (SMR), which in turn are fitted to the target object in predetermined locations with the use of sphere mounts, such as is shown at in FIG. 1. The hollow steel balls include a circular opening or aperture in an exterior surface, through which laser light enters the ball and is reflected back along an incident angle to the source by the internally mounted retro-reflector. Surrounding the circular opening or aperture is a cylindrical hood, configured to reduce or eliminate unwanted glare and to protect the glass optical retro-reflector from breakage. Each sphere mount is typically cylindrical or disc shaped, and include a recessed conical nest in one surface which holds and locates the retro-reflector ball, usually with the aid of a magnet. The opposite surface of the sphere mount is either flat to within a predetermined tolerance or includes a base shank, configured for seating within a correspondingly sized bore on the target object, permitting the sphere mount to be located at a known position on the target object. For high tolerance applications, a sphere mount having a three-point conical nest, commonly referred to as a kinematic mount, may be utilized.
Conventional sphere mounts such as those shown in FIG. 1 come in a variety of standard sizes, such as 0.500″, 0.875″, and 1.500″ and have a hemispherical range of illumination acceptance, which is generally limited by the mechanical interaction between the edge of the circular opening or aperture in the retro-reflector ball and the upper surface of the sphere mount. FIG. 4 illustrates how the retro-reflector ball in a conventional sphere mount may rotate through a vertical arc of 180° and may rotate 360° about the central axis within the conical nest of the sphere mount.
In some applications, the laser tracker or illumination source may not be disposed within the hemispherical range of illumination acceptance defined by the upper surface of the sphere mount, but rather, slightly below the lower boundary. For these applications, it is known to employ sphere mounts having an axially perpendicular recess between the base of the conical nest and the circumferential perimeter, such as is shown in FIG. 5. Illustrated in phantom in FIG. 5, a retro-reflector ball placed in such a modified sphere mount may rotate through a vertical arc angle greater than 180° when radially aligned with the perpendicular recess, by seating the cylindrical hood within the recess, providing an extending illumination acceptance range.
Vertical arc angles of illumination acceptance from 0° to 180° are considered “positive” angles while vertical arc angles of acceptance greater than 180° (i.e. below the horizon of the sphere base, are considered “negative” angles. However, as can be seen in FIG. 5, a portion of the circular opening or aperture to the ball mounted retro-reflector is occluded by the modified sphere mount, reducing the effective size of the circular opening or aperture and rendering measurement readings more difficult to obtain.
Accordingly, there is a need in the portable precision measurement industry for a modified sphere mount design which will provide a maximum illumination acceptance range for a ball mounted retro-reflector greater than 180°, and which will not occlude the retro-reflective elements and illumination source.